P1270: IMMUNE REGULATORY CELLS IN B-CELL LYMPHOMAS: SIGNIFICANT HETEROGENEITY ACROSS HISTOLOGIC TYPES.
Authors: M. STAVROULAKI, A. DAMIANAKI, K. CHAIKOU, A. KAVIDOPOULOU, P. VERGINIS, C. KALPADAKIS
Journal: HemaSphere (2022)

Background: Immune regulatory cell populations play a fundamental role in tumor development and tumor immune evasion. Myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs) considered as dominant suppressor cells which are significantly enriched in blood, bone marrow (BM), or tumor sites of patients with hematologic malignancies and often associate with high tumor burden, adverse clinical features, and outcome. Studies in lymphoma patients however, have generated conflicting results, regarding the function and phenotype of MDSC and Tregs mainly due to large heterogeneity of the disease as well as due to the lack of a unanimous nomenclature of these cell subsets in human samples. Furthermore, the molecular mechanisms via which MDSCs and Tregs contribute to clinical outcomes of disease and how they interfere with response to therapy remain elusive. Aims: To investigate the prevalence of immune suppressive population in B-cell lymphomas according to histologic, subtype providing the basis for further analysis of their biologic properties and association with clinical features and outcome of specific lymphoma subtypes. Methods: Peripheral blood (PB) and BM samples were collected from healthy individuals (n= 22) and lymphoma patients (n=50). Specifically, lymphoma subtypes were as follows:Marginal Zone (MZL, N=19), Diffuse Large B-cell Lymphoma (DLBCL, N=18) and Follicular Lymphoma (FL, N=13). Peripheral blood mononuclear cells (PBMCs) were isolated with ficoll gradient and subjected to flow cytometry analysis. MDSC characterization was based on HLADR-/lowCD11b+CD33- expression with CD14+CD15- for M-MDSCs and CD14-CD15+ for PMN-MDSCs. In addition, Treg cells were denoted as CD4+CD25highCD127-Foxp3+. Expression of PDL1 and PD1 on MDSCs and Tregs respectively, was also determined by flow cytometry. M-MDSCs and PMN-MDSCs were sorted from BM samples, RNA was extracted in order to subjected to transcriptomic analysis (RNAseq). Informed consent was obtained. Results: Frequencies of MDSCs were significantly increased in PB of all lymphoma patients compared to healthy individuals, with MZL subtype exhibiting the most prominent increase of MDSCs (p<.0001). Notably numbers of M-MDSCs were increased in FL and MZL patients and to a lesser extend in DLBCL patients while PMN-MDSCs were elevated in all subtypes with MZL displaying the higher frequencies. Regarding the Treg cells, although all lymphoma patients exhibited increased absolute numbers of CD4+CD25highCD127- Treg cells, Foxp3-expressing Tregs were significantly increased in the PB of DLBCL and MZL patients, but not in FL patients. Of interest, CD4+ and CD8+ T cells in the PB of DLBCL patients exhibited increase expression of PD1 compared to respective cell subsets from healthy individuals. Summary/Conclusion: Our findings comprehensively demonstrate enhanced numbers of MDSC and Treg immune suppressive cells in the PB of B-cell lymphoma patients while an increased heterogeneity was observed among the different histological lymphoma types. Functional characterization of MDSCs and Tregs by transcriptomic (RNAseq) and proteomic analysis are ongoing.